CuIn1-xGaxSe2 Research Articles

CIGS Mini‐Modules with Dispensed Metallization on Transparent Conductive Oxide Layer

This article presents the application of a parallel dispensing device for low‐temperature silver paste metallization on transparent conductive oxide (TCO) layers of Cu(In1 − xGax)Se2 (CIGS) substrates as an alternative metallization technology to screen printing and inkjet printing. A curing variation experiment is performed to analyze the effect of different curing conditions on the resulting contact resistivity of the metal grid. Contact resistivity values below 5 mΩcm2 are achieved. Furthermore, CIGS mini‐modules are metallized with three different low‐temperature paste formulations obtaining a record core finger geometry of 25 μm and an average optical aspect ratio of 0.46 using 25 μm nozzle openings. The dispensed metal grid on the TCO layers achieves the comparable current density values of jsc = 32.2 mA cm−2 and the open‐circuit voltages values per cell of Voc = 672 mV as the screen printed metal grid on CIGS mini‐modules and, hence, a nominal power of 2.05 W. The metal grid enables the use of broader cell widths compared with grid‐free CIGS samples and results in a reduced dead area.

Opto-electrical characterization of quaternary sputtered copper indium gallium selenide nanorods via glancing angle deposition

Nanorod arrays have become an attractive alternative to their thin film and bulk counterparts in photovoltaic and photoconductivity research. This is mainly attributed to their superior optical and electrical properties. Light trapping and unique bandgap geometries in vertically aligned nanostructures result in high optical absorption and provide enhanced carrier collection by utilizing a fully depleted p-n junction between the anode and cathode via an isolated “capping” construction. The combination of these two features leads to the development of high efficiency nanostructured devices that can be utilized in solar cells and photodetectors. Optical absorption properties, geometry, and opto-electrical properties of nanorod arrays of CuInxGa(1−x)Se2 (CIGS), a p-type semiconductor with a wide bandgap ranging from 1.0 to 1.7 eV, are compared to their thin film counterparts. Utilizing a radio frequency sputtering system, a quaternary target, and glancing angle deposition technique, both isolated vertical arrays of CIGS nanorods and “core-shell” devices were fabricated, while conventional film devices were fabricated by normal incidence deposition. Scanning electron microscopy images indicated a successful growth of CIGS nanorods. Optical absorption and opto-electrical performance were found to be strongly improved by the presence of the isolated nanorod structures through spectroscopic reflectometry and responsivity testing under a solar simulator.

Detrimental copper-selenide bulk precipitation in CuIn1-xGaxSe2 thin-film solar cells. A possible reason for the limited performance at large x?

In this paper, we propose a possible explanation of the limited CuIn1-xGaxSe2 (CIGSe) performance at high Ga contents, related to the properties of detrimental copper selenide (CuδSe) secondary phases. We study CIGSe layers at different x, by means of X-ray diffraction analyses, Raman and energy dispersive X-ray spectroscopy. Our results reveal that for high Ga contents, CuδSe secondary phases either precipitate at the interfaces between grains or remain within the layer as intra- or inter-grain clusters, deteriorating the electronic properties of the absorber layer. On the contrary, for low Ga contents, the copper selenide segregates at the surface of the CIGSe and hence it can be easily removed by KCN surface etching. To understand the CuδSe precipitation within the bulk of the film for a high Ga ratio, we also investigate In-free CGSe samples at low and high Cu content. Our observations demonstrate that i) CuδSe has a preferential grain orientation to segregate and ii) the Cu-enriched regions within the bulk of the high Ga content films are more a kinetic than a thermodynamic issue.

Ultrathin CIGSe Solar Cells with Integrated Structured Back Reflector

To reduce the thickness of CuIn1‐xGaxSe2 (CIGSe) solar cells, light management concepts must be used. One concept is the combination of an Al/indium1‐tin1‐oxide (ITO) reflector/contact and an optical scattering element at the back side of the device. Herein, nanostructured substrates, obtained by laser interference lithography and lift‐off process, are covered with an Al/ITO back contact. Structure dimensions are a pitch of 1.96 μm and heights of 100–700 nm. Solar cells with 600 nm CIGSe absorber having this integrated structured back reflector exhibit an increase in JSC of up to 3.2 mA cm−2 compared with the Mo reference sample of identical thickness. To understand this optical gain, 3D finite‐difference time‐domain (FDTD) simulations are performed. To this end, a model is built to simulate structure formation. It is shown that locally enhanced optical absorption can be achieved by the integrated structured back reflector. It is also shown that parasitic absorption in the back ITO diffusion barrier needs to be minimized.

Temperature‐dependent current–voltage and admittance spectroscopy analysis on cesium‐treated Cu (In1 − x,Gax)Se2 solar cell before and after heat‐light soaking and subsequent heat‐soaking treatments

AbstractRecently, we demonstrated the positive effects of heat‐light soaking (HLS) and subsequent heat‐soaking (HS) on cesium fluoride (CsF) treated Cu(In1‐x,Gax)Se2 (CIGS) solar cells. However, the role of defects formation and its influence on the electronic properties have not been analyzed. With this motivation, here, we analyzed the electronic properties of CsF‐free and CsF‐treated CIGS solar cells before and after HLS and subsequent HS treatments using temperature‐dependent current–voltage (J–V–T), admittance and low‐temperature capacitance–voltage (C–V) measurements. We noticed that CsF‐treated CIGS solar cells form a minority carrier trap level after HLS. The subsequent HS treatment was found to be beneficial to compensate this defect level. The admittance measurement showed a shift of the shallow energy position to a higher value after HLS and subsequent HS treatments, irrespective of Cs incorporation. This is expected to be due to the formation of a secondary diode toward the CIGS/molybdenum contact. The positive and negative effects of HLS and subsequent HS treatments on CsF‐treated CIGS solar cell are discussed using low‐temperature C–V measurements. By optimizing the HLS and HS processes, CsF‐treated CIGS solar cells yielded total efficiencies of over 20%.

Single junction-based thin-film CIGS solar cells optimization with efficiencies approaching 24.5 %

This research paper proposes a numerical analysis of a new Cu(In1-xGax)Se2 (CIGS) based solar cell integrating an ultrathin absorber layer. The CIGS model is performed from the theoretical and experimental investigations taking into account the physical properties, dimensions, and thicknesses of the different layers by using Atlas SILVACO-TCAD tools. To improve the overall efficiency, magnesium fluoride (MgF2) thin film is used as the antireflective coating to maximize the photocurrent from the incident light intensity. The influence of the minority carrier lifetime, density of state, and interface defect density in the CIGS on the PV performance are investigated under the AM1.5 spectrum, 300 K. The novelty of the proposed solar cell consists of a CuInSe working as a back-surface field layer (BSF). The simulation results point out that the CuInSe BSF layer has a strong effect on cell performance. The use of ultrathin absorber layers (≤1 μm) can provide a new solution for recent advanced research on this topic. The investigation illustrates potential results for conversion efficiency improvement up to 24.5 % (Voc =751 mV, Jsc =41.44 mA/cm2, FF = 78.63 %).

Flexible and Semi‐Transparent Ultra‐Thin CIGSe Solar Cells Prepared on Ultra‐Thin Glass Substrate: A Key to Flexible Bifacial Photovoltaic Applications

AbstractFor applications to semi‐transparent and/or bifacial solar cells in building‐integrated photovoltaics and building‐applied photovoltaics, studies are underway to reduce the processing cost and time by decreasing the thickness of Cu(In1−x,Gax)Se2 (CIGSe) absorber to the ultra‐thin scale (≤500 nm). To dynamically and affordably meet the growing demand for electric power, daylighting, and architectural aesthetics of buildings in urban area, flexible semi‐transparent ultra‐thin (F‐STUT) CIGSe solar cells are proposed on flexible ultra‐thin glass (UTG) and compared with rigid semi‐transparent ultra‐thin (STUT) CIGSe solar cells fabricated on soda‐lime glass (SLG). At all the tested deposition temperatures of CIGSe, the F‐STUT CIGSe solar cells exhibit superior performance compared to the rigid STUT CIGSe solar cells. Furthermore, through realistic measurement under ≈1.3‐sun illumination, maximum bifacial power conversion efficiency of 11.90% and 13.23% are obtained for SLG and UTG, respectively. The major advantages of using UTG instead of SLG are not only the intrinsic characteristics of UTG, such as flexibility and high transmittance, but also collateral benefits such as the larger CIGSe grain size at the deposition temperature, better CIGSe crystalline quality, more precise controllability of the alkali element, and reduced thickness of the interfacial GaOx layer, which enhance the photovoltaic parameters.

Physical properties of CuIn0.7Ga0.3Se2 ingot and thin films prepared by one-step rf-magnetron sputtering from single-target material

Nowadays, large area CuIn1-xGaxSe2 (CIGS) thin film fabrication by one-step radio-frequency magnetron sputtering is still facing difficulties and challenges in uniformity of composition and structure. This study illustrates how we can prepare polycrystalline thin layers of CuIn0.7Ga0.3Se2 on glass and on F-doped SnO2/SLG substrates using single quaternary target. It was found that using direct reaction of stoichiometric proportions of pure elements, we can make high quality CIGS powder to prepare sputtering target. X-ray diffraction study of CIGS powder revealed all possible orientations for the CIGS material. Compositional analysis of the obtained CIGS crystal surface was also investigated. To investigate the defect chemistry, Cathodoluminescence (CL) measurements at different temperatures were carried out. The structural, compositional, morphological, and electrical properties of the as-deposited films were systematically investigated by XRD, SEM, AFM, and four-point probe method. By adjusting the processing parameters at high substrate temperature TS = 300 °C, a CIGS thin films with a preferential orientation along (112) direction which exhibited single chalcopyrite phase were obtained. The optical transmission showed energy gap value up to 1.19 eV and rather sharp absorption fronts which is suitable for high efficient solar cells.

Measurement of shunt resistance and conduction band offset in Cu(In, Ga)Se2 solar cells through joint analysis of temperature and intensity dependence of open-circuit voltage and photoluminescence

Joint analysis studies of open-circuit voltage and photoluminescence intensity (PL-I) are reported for CuIn1−xGaxSe2 (CIGSe) solar cells. A range of compositions are investigated, including constant x = 0.35 and x = 0.55 as well as a graded composition profile having a minimum of x = 0.25. Both the open-circuit voltage and PL-I are measured as functions of temperature and illumination intensity. With these two measurements, a full model-based fitting of the temperature and illumination dependence allows extraction of the effects of window layer band offset and shunt resistance, in addition to bulk and interface recombination parameters. To quantitatively analyze the two distinct measurements jointly, the absolute PL-I is measured to obtain quasi-Fermi-level splitting.

Enabling Low-Temperature Deposition of High-Efficiency CIGS Solar Cells with a Modified Three-Stage Co-Evaporation Process

CuIn1–xGaxSe2 (CIGS) absorber layers with good crystallinity and large grain sizes deposited at low substrate temperatures are essential to the development of CIGS solar cells. The main challenges ...

Characterization of lattice parameters gradient of Cu(In1-xGax)Se2 absorbing layer in thin-film solar cell by glancing incidence X-ray diffraction technique

In or Ga gradients in the Cu(In1-xGax)Se2 (CIGS) absorbing layer lead to change the lattice parameters of the absorbing layer, giving rise to the bandgap grading in the absorbing layer which is directly associated with the degree of absorbing ability of the CIGS solar cell. We tried to characterize the depth profile of the lattice parameters of the CIGS absorbing layer using a glancing incidence X-ray diffraction (GIXRD) technique, and then investigate the bandgap grading of the CIGS absorbing layer. When the glancing incident angle increased from 0.50 to 5.00°, the a and c lattice parameters of the CIGS absorbing layer gradually decreased from 5.7776(3) to 5.6905(2) Å, and 11.3917(3) to 11.2114(2) Å, respectively. The depth profile of the lattice parameters as a function of the incident angle was consistent with vertical variation in the compositionof In or Ga with depth in the absorbing layer. The variation of the lattice parameters was due to the difference between the ionic radius of In and Ga co-occupying at the same crystallographic site. According to the results of the depth profile of the refined parameters using GIXRD data, the bandgap of the CIGS absorber layer was graded over a range of 1.222–1.532 eV. This approach allows to determine the In or Ga gradients in the CIGS absorbing layer, and to nondestructively guess the bandgap depth profile through the refinement of the lattice parameters using GIXRD data on the assumption that the changes of the lattice parameters or unit-cell volume follow a good approximation to Vegard’s law.

Luminescence properties of pulsed laser deposited CuInxGa1−xSe2 films

Pulsed laser deposition (PLD) of CuIn1−xGaxSe2 (CIGS) provides a low cost, single-step process via which stoichiometric, high quality thin films for light harvesting applications can be produced. Little is known about the optical properties of PLD-deposited CIGS and how they compare with the respected properties of the well-studied evaporated or sputtered CIGS films. We report herein a systematic spectroscopic investigation, probing the influence of PLD deposition temperature on the energetics and dynamics of emission from CuIn0.7Ga0.3Se2 films. Variable-temperature steady-state and time-resolved photoluminescence in combination with Gaussian lineshape analysis allow us to unravel the contribution and nature of three main radiative channels, with the high energy one associated with electronic and two lower energy ones with defect levels. The analysis show that the band-edge luminescence grows at the expense of defect emission as PLD temperature increases in the 300 °C–500 °C range. This is further supported by: (i) The dramatic increase of the band-edge recombination lifetime from 30 to 180 ns, (ii) The quenching in the carrier trapping rate from 0.25 ns−1 to 0.09 ns−1 as growth temperature increases. The results correlate well with structural and electrical characterization studies reported previously on PLD-grown CIGS and rationally interpret the improvement in their optoelectronic properties as PLD deposition temperature increases .

Composition-Dependent Passivation Efficiency at the CdS/CuIn1- x Gax Se2 Interface.

The bandgap of CuIn1- x Gax Se2 (CIGS) chalcopyrite semiconductors can be tuned between ≈1.0 and ≈1.7 eV for Ga contents ranging between x = 0 and x = 1. While an optimum bandgap of 1.34 eV is desirable for achieving maximum solar energy conversion in solar cells, state-of-the-art CIGS-based devices experience a drop in efficiency for Ga contents x > 0.3 (i.e., for bandgaps >1.2 eV), an aspect that is limiting the full potential of these devices. The mechanism underlying the limited performance as a function of CIGS composition has remained elusive: both surface and bulk recombination effects are proposed. Here, the disentanglement between surface and bulk effects in CIGS absorbers as a function of Ga content is achieved by comparing photogenerated charge carrier dynamics in air/CIGS and surface-passivated ZnO/CdS/CIGS samples. While surface passivation prevents surface recombination of charge carriers for low Ga content (x < 0.3; up to 1.2 eV bandgap), surface recombination dominates for higher-bandgap materials. The results thus demonstrate that surface, rather than bulk effects, is responsible for the drop in efficiency for Ga contents larger than x ≈ 0.3.

Effect of Annealing Time and Heat Flux on Solvothermal Synthesis of CIGS Nanoparticles

Abstract CuInxGa(1-x)Se2 (CIGS) is a renowned absorbing material in solar cell application due to its wide and tunable direct bandgap with high absorption coefficient. The highest efficiency achieved on lab scale device is 22.6% which is competitive to Si based solar cells. CIGS nanoparticles are prominent materials for solar photovoltaic applications due to its high surface to volume ratio and for its use in relatively low processing temperature. In the study, CIGS nanoparticles are synthesized using modified solvothermal method in which the size and agglomeration of CIGS nanoparticles are controlled by introducing ammonium halides. The formation mechanism of CIGS nanoparticles while heating at 230 °C in hot air oven and in muffle furnace for various time durations are studied and analysed in detailed. The crystal phase and crystallite size are evaluated using X-ray diffraction and the phase is confirmed by Raman spectroscopy. The synthesized CIGS nanoparticles from 5 hours and 7 hours of reaction time exhibited tetragonal chalcopyrite crystal structure with an average crystallite size of 41 nm in hot air oven and 125 nm in furnace. The optical band gap of the CIGS nanoparticles measured by UV-Vis-NIR spectroscopy is found to be 1.52 eV for 5 hours and 1.56 eV for 7 hours of reaction time in both oven and furnace prepared samples, that indicates the quality of material is high and could be used for photovoltaic device fabrication.

Theoretical investigation of electronic and optical properties of the CuIn1-x GaxSe2: Ab initio calculation

The controversial materials using in solar cell, Copper-Indium-Gallium-Selenide (CIGS) is highly favorable for solar power systems, due to its large optical absorption coefficient, long term stability, low cost and high conversion efficiency. The electronic and optical properties of CuIn1-xGaxSe2, are calculated using the density functional theory (DFT) implemented on Wien2k code with GGA + mBJ approximation, for different concentrations of Ga. Moreover, we compare the physical proprieties between CuInSe2 CuIn0.75Ga0.25Se2, CuIn0.50Ga0.50Se2, CuIn0.25Ga0.75Se2, CuGaSe2 in the wavelength range. According to the picks obtained, we can affirm that the CIGS is a good candidate for photovoltaic applications, especially the CuIn0.75Ga0.25Se2 because it’s much more absorptive and productive, with an optical absorption that reaches 104 in visible range.

Effect of MoSe2 on the performance of CIGS solar cells

A CuIn1-xGaxSe2 (CIGS) thin film solar cell model with MoSe2 transition layer was established, using SCAPS-1D software. The influence of MoSe2 interface layer formed between absorption layer CIGS and the back contact Mo on the solar cell performance was investigated. By changing the doping concentration, thickness and bandgap of MoSe2 layer, it is found that the MoSe2 and the variation of parameters have a significant effect on the electrical characteristics and photovoltaic parameters of CIGS thin film solar cells. Based on the energy band, the interfaces of Mo/MoSe2 and MoSe2/CIGS are analyzed. It is considered that Mo/MoSe2 is a Schottky contact, MoSe2/CIGS is an ohmic contact. When suitable parameters of MoSe2 layer are formed into the interface, it will provide a new path for designing CIGS solar cells with thinner absorption layer.

Depth profile crystal orientation determination of Cu(In1−xGax)Se2 thin films by GIXRD method applying skin depth theory

In this study, Cu(In1−xGax)Se2 (CIGS) thin film was deposited on molybdenum (Mo) coated glass substrate by thermal co-evaporation technique and depth profile crystal orientation of the this film was examined by grazing incidence X-ray diffraction (GIXRD) method. The crystal structure of the CIGS thin film was determined within 20° to 80° (2θ) scanning range at 0.5° to 15° incidence angle of X-ray. Cross section images were investigated by scanning electron microscope measurements and these measurements showed that bilayer Mo thickness and CIGS thickness are approximately 480 nm and 2 µm, respectively. The surface roughness of films investigated by atomic force microscopy (AFM) and average roughness Ra was found 11.07 nm. According to GIXRD measurements; the interdiffusion of the constituent elements and their effect on the crystal structure were defined both electromagnetic field penetration (skin depth) and mass attenuation viewpoints, and also these results were supported by energy dispersive spectroscopy measurements. As a result, it was seen that the GIXRD method can be used with ease to define the crystal phase homogeneity and depth profile characterization of thin film volume applying skin depth theory.

Simulation of graded bandgap on backwall superstrate CIGS solar cells with MoOx electron reflection layer

A model of backwall superstrate CuInxGa(1−x)Se2 (CIGS) solar cell with MoOx as an electron reflection layer has been investigated by Solar Cell Capacitance Simulator (SCAPS). The influence of the CIGS absorber with different thicknesses on the performance of the CIGS solar cells with a flat bandgap structure is carefully analyzed. When the CIGS thickness is 400 nm, the device with 10 nm MoOx layer has the highest efficiency of 8.24%. To further increase the efficiency, a graded bandgap structure, near the MoOx layer, has been established, and the efficiency can be improved from 8.24% to 15.01% when the maximum bandgap value and the length of the graded region are 1.6 eV and 240 nm, respectively. The causes of efficiency enhancement by this graded bandgap structure are then studied. With an additional graded bangap structure in SCR, close to the CdS layer, the efficiency is slightly improved from 15.01% to 15.15%. Finally, the simulation results show that the efficiency can be increased from 15.15% to 16.26% when the thickness of MoOx is reduced from 10 nm to 1 nm.

Physical properties of electrodeposited CIGS films on crystalline silicon: Application for photovoltaic hetero-junction

P-type CIGS (CuIn1-xGaxSe2) thin films are electro-deposited on a p-type c-Si substrate with a galvanostatic mode to form CIGS(p)/c-Si(p) hetero-junction. The Ga content is varied up to x = 30%. The physical properties of formed CIGS films are characterized by XRD, SEM, EDS and UV–Visible spectroscopy. With x = 30%, we obtain a single chalcopyrite phase of CIGS with a tetragonal crystal structure, a high crystallinity, an orientation toward the (112) direction and a band gap energy of 1.40 eV.AM1.5 J-V performed on the CuI0.7G0.3Se2/c-Si hetero-junction reveals interesting photovoltaic parameters with an efficiency of 3.75%. In addition, using the energy diagram of the hetero-junction calculated with the Anderson model, we show that it could play a dual role when combined to a c-Si cell in a Ag–Al/c-Si(n+)/c-Si(p)/CIGS(p)/Al new architecture. Therefore, in addition to its interesting photovoltaic parameters, this hetero-junction can substitute the BSF.

Impact of Host Composition, Codoping, or Tridoping on Quantum-Cutting Emission of Ytterbium in Halide Perovskite Quantum Dots and Solar Cell Applications.

Recently, various lanthanide ions (Ln3+) have been successfully doped into perovskite quantum dots (PQDs), and the quantum-cutting emission of 2F5/2-2F7/2 for Yb3+with a measurable inner efficiency of more than 100% has been discovered and applied as the luminescent converter of solar cells, which has opened a new branch for the application of PQDs. In this work, to further improve the quantum-cutting efficiency of Yb3+, the codoping and tridoping methods were used to improve the quantum-cutting emission of PQDs. The Yb3+-Ln3+ (Ln = Nd, Dy, Tb, Pr, Ce) pair-doped CsPbClxBryI3-x-y PQDs were fabricated, with all displaying excitonic emission, narrow-band emission of Ln3+ ions, and quantum-cutting emission of Yb3+ ions. It was interesting that Yb3+-Pr3+ as well as Yb3+-Ce3+ pairs could effectively sensitize the emission of Yb3+, owing to Pr3+ and Ce3+ ions offering intermediate energy states close to the exciton transition energy of the PQDs. After host composition optimization and tridoping investigation, overall emissions with a 173% photoluminescence quantum yield (PLQY) were obtained in the Yb3+-Pr3+-Ce3+-tridoped CsPbClBr2 PQDs. Then, the tridoped PQDs were designed as the down-converter for CuIn1-xGaxSe2 (CIGS) as well as the silicon solar cells, which leads to an enhancement of the power conversion efficiency (PCE) of as high as ∼20%. The modified CIGS was further employed to charge the smart mobile phone, which could largely shorten the charging time from 180 to 150 min. This finding is of great significant for expanding the application fields of the impurity-doped PQDs.